Electric utility companies have primary and secondary lines for distributing electric power. Primary lines are used to deliver electric power to customer service transformers, which provide electricity to designated geographic areas. Utilities protect these primary lines from overload with circuit breakers or other types of electric current interrupters which trip “off” when a fault condition occurs. (For the sake of brevity, all types of circuit interrupters will hereinafter be referred to as “circuit breakers”; however, it will be known and understood that the use of the term “circuit breaker” includes circuit breakers, cutouts and other types of circuit interrupters.) In the “off” state, the primary line sees an “open” circuit at the location of the “off” circuit breaker, and current will not flow in the circuit. When a fault condition occurs, all customers whose source of electric power is dependent upon the primary line can be without electric service until the cause of the fault is located and repaired. In order to confine power outage to a given area, circuit breakers are connected to the primary lines at or near the customer service transformers. Fuse links installed in the circuit breakers are designed to “blow” (or “open”) when a fault occurs; i.e., when the current passing through the fuse link exceeds a predetermined value. When the fuse link opens, it isolates a faulted customer service transformer from the primary line and prevents additional power failures to other areas served by the primary line.
Utilities generally prefer to use enclosed circuit breakers rather than open-type circuit breakers because the enclosed type requires less spacing for the breaker's components. In congested areas, the height of a utility pole is typically shared by power lines and cables from utilities, communications, and television companies, and increased pole height results in increased operations costs. The use of enclosed circuit breakers is therefore one method for reducing operating costs, because pole heights do not need to be increased to accommodate all the material that is installed onto the poles.
One such enclosed circuit breaker in use is the Positect® enclosed insulator assembly. There are other circuit breakers of this type, as well, sold under other brands. For the sake of simplicity, the term “Positect®-type enclosed insulator assembly” will be used to denote all of these. These enclosed insulator assemblies can be used as either a utilities distribution circuit breaker or as a disconnect switch, depending on whether a cutout fuse tube assembly or disconnect tube assembly is utilized within them.
A Positect®-type enclosed insulator assembly comprises a hollow, generally cylindrical porcelain insulator body having a manually removable handle formed from the same material as the body. A generally central co-axial tube formed from a plastic material such as Delrin® resin is supported within the porcelain body, and encloses a manually insertable/removable, generally tubular cutout fuse holder (for operation as a circuit breaker) or manually insertable/removable disconnect switch holder (for operation as a disconnect switch) that is attached to the handle.
Electrically conductive contact surfaces on the insertable holders couple to electrically conductive tulip-type contacts associated with the generally central co-axial tube to place the holder in circuit with the power lines.
As a circuit breaker, the enclosed insulator assembly is typically used for four basic distribution utility applications: 1) protecting distribution transformers (single units or banks), 2) protecting underground risers, 3) protecting capacitor banks, and 4) sectionalizing branch circuits subjected to damage by tree branches. When used to protect transformers or underground risers, a universal fuse link is typically installed within the cutout fuse holder. When used to protect capacitors, an indicating fuse link is typically installed in the cutout fuse holder.
As a disconnect switch, the enclosed insulator assembly is typically used to isolate a geographic area served by the line, and is required to interrupt primary circuits while energized at the required voltage. A gloved utility lineman can isolate an area from the circuit by manually pulling out the handle with attached disconnect switch holder (all referred to as disconnect bayonet) from the body without first de-energizing the entire circuit. When field conditions change (e.g., due to tree growth on private property), an enclosed disconnect switch used for sectionalizing the branch circuit can be converted to a circuit breaker by replacing the disconnect switch holder within the central tube with a cutout fuse holder.
The enclosed design of these insulator assemblies is intended to offer a high degree of safety; their high interrupting capability and their shielded electrically-live parts keep the utility lineman safe and provide animal protection as well. The design is also intended to reduce and keep exhaust blast away from the utility lineman. When connected to these enclosed circuit breakers, transformers, underground risers, and capacitor banks energized at rated voltage and current can be de-energized by a utility lineman by manually pulling out the handle with attached cutout fuse tube assembly (all referred to as cutout bayonet) from the porcelain insulator body. When connected to these enclosed disconnect switches, an energized branch circuit at rated voltage and current can be sectionalized by a utility lineman by manually pulling out the disconnect bayonet from the porcelain insulator body.
The design of the enclosed insulator assembly is also directed to suppressing the electric arc that can occur as either of the bayonets is pulled out from the insulating body. As the attached cutout fuse holder (or attached disconnect switch holder) is removed from the central tube by the pulling out of the handle, an arc can be created between the holder and the central tube from which it is being withdrawn. The arc must be extinguished as quickly as possible. A “load interrupter” is accordingly coupled to the bottom the cutout fuse holder (or disconnect switch holder) to extinguish the arc as the holder is withdrawn. The load interrupter is shaped to compress the arc and the dielectric gas given off as the arc contacts the central tube. As the load interrupter is pulled through the insulator body, it compresses the arc against the central tube.
The Positect™-type enclosed insulator assembly is made from porcelain. Porcelain is heavy and brittle which may cause a long installation time or breakage during transport, handling, or installation. These cause operating costs to increase for a utility. The enclosed insulator assembly also uses lead to secure a threaded cylinder into the handle. Lead is heavy and toxic. Over time, the enclosed insulator assembly is subjected to a variety of weather conditions and air contamination, causing corrosion to occur between the insulator terminals and tube fittings which make it difficult to remove the bayonet.